Water quality varies across the UK and affects choice of hot water system, as well as water heater maintenance. The hardness of the water and the material of the tank/cylinder causes different reactions within the unit.

Fresh water carries oxygen and is a major contributor to corrosion in heating and hot water systems. The rate of corrosion is affected by the hardness of the water. But why? Firstly, we need to understand the difference between hard and soft water.

For more in depth training on water heater maintenance take a look at our CIBSE accredited CPD on Best Practice in Domestic Hot Water.

What’s the difference between hard water and soft water?

The hardness of water is measured in parts per million (ppm) of mineral deposits. This is caused by the geology of the region.

What is hard water?

Hard water areas generally have > 280ppm. Hard water forms when water passes through limestone and chalk in the ground which are full of calcium and magnesium carbonates.

What is soft water?

Soft water areas generally have < 100ppm. Soft water comes from areas across the country that are formed of hard igneous rock, like granite that rain water runs over. Scotland, South West and North West England, and West of Wales are typically soft water areas. Soft water is pure and has low electrical conductivity.

But why does this matter? 

The conductivity of the water effects the method of protection you choose for your water heater. 

Choosing anodes to protect your water heater from corrosion - comparing sacrificial anodes and powered anodes

What is a sacrificial anode?

A sacrificial anode by virtue of its name ‘sacrifices’ itself for the benefit of the hot water cylinder. Using the principles of electrochemical processes when two different metals are immersed in water one will corrode away to protect the other. Sacrificial magnesium anodes are used in steel hot water cylinders acting as the less noble metal and corrodes to protect the steel lining of the cylinder (cathode).

Sometimes the quality of water prevents magnesium anodes from corroding. These water conditions are typically soft water. The low electrical conductivity in soft water reduces electrical flow from anode to cathode (cylinder shell) through the water. This stops the anode from corroding and therefore won’t protect the cylinder shell.

When inspecting the anode in these applications it could appear in excellent condition. However, this may be an indication that the anode is not working, and the heater is at risk.

What is a powered anode?

Powered anodes, or impressed current anodes, are non-sacrificial. The metal of the anode is titanium and theoretically will last the lifetime of the hot water cylinder.

Powered anodes use an electrical supply to produce a very low current into the water to replace the electrolytic current produced by sacrificial anodes.

It is good practice to remove the sacrificial anode (if already installed) from the hot water cylinder to maximise protection.

Which anode protection should I use for my water heater or calorifier?

It depends on the area of the UK you are in and your choice of hot water heater.

Things to consider when choosing anode protection for your water heater:

Hardness of the water

Sacrificial anodes often don’t work in soft water areas due to the low electrical conductivity. But a powered anode in certain soft water areas may also have no effect in protecting the cylinder shell as the conductivity of the water, which the powered anode requires, may still be too low.

Material of the hot water cylinder

Many hot water tanks and calorifiers are constructed in steel with a glass lining, like most of our Dorchester water heaters and Powerstock calorifiers. The glass lining stops the water contacting the metal protecting it from corrosion. However this is not enough alone. Most glass lined water heaters and calorifiers will be provided with anodes as standard items. 

But stainless steel cylinders like our Halstock calorifiers and Dorchester DR-XP direct fired water heater require no anode protection as they are not susceptible to corrosion. This is why they are often used in soft water areas. 

Site conditions

If the site is prone to electrical supply failures then powered anodes should not be used as without a power supply the hot water cylinder will not be protected from corrosion.

Cost and installation

Electrical anode protection can be more expensive initially due to a higher purchase price and the additional electrical connections that are needed for installation. However, over the lifetime of the unit they could be more cost effective than maintaining and replacing sacrificial anodes. 
Again, all this needs to be considered in relation to the site and individual project.


Electrical supply and connections are needed for powered anodes. However powered anodes come with an indicator red or green LED to help you identify the status of the protection system. 

Sacrificial anodes require maintenance and will corrode at different rates depending on the location. Regular inspection is required by an engineer to check the state of the anodes. You will need to learn how quickly the anodes deteriorate and therefore how often they need changing. 

Remember, the sole purpose of an anode is to protect against corrosion and does not have any effect on reducing scale or legionella in the hot water cylinder. 

A stainless steel calorifier solution for soft water areas

Our Halstock hot water calorifier is manufactured in Acerinox duplex stainless steel.

So as well as being corrosion resistant and easy to maintain the mixed use of metals create a stronger stainless steel that prolongs the life of the cylinder. This is backed up with a 5-year cylinder warranty. Find out more about Halstock stainless steel calorifier.

A stainless steel water heater for soft water areas

The Dorchester DR-XP range of pre-mix, condensing water heaters are designed to meet the needs of smaller projects in soft water areas such as Scotland and the South West.

Dorchester DR-XP stainless steel direct fired water heater

Learn more with a CIBSE-accredited CPD

Module 4 of our CIBSE accredited CPD seminar covers water treatment and protecting your hot water calorifiers.

With such a big topic this CIBSE accredited CPD has been broken down into bitesize chunks. You pick and mix the modules you want to learn inducing hot water system design, hot water sizing, legislation, safety, and water treatment. See the full course outline.

CIBSE accredited CPD on best practice hot water systems